The present invention relates to a color image reading apparatus used for a color copier, a color scanner, a color facsimile system and the like and, more particularly, to a color image reading apparatus for obtaining a color image signal upon switching of a color separation filter arranged along an optical passage for imaging an object image on an image sensor.
In one method for reading a color image printed on a document, at least one color separation filter is arranged along an optical passage for guiding and focusing light reflected by a document surface onto an image sensor constituted by a photosensor array, and the color separation filter is mechanically switched. A color image reading apparatus described in Japanese Patent Disclosure (Kokai) No. 60-263564 is known. In this apparatus, a color separation filter has R (red), G (green), and B (blue) filter elements linearly formed on a single plane. The filter is moved by an actuator in a direction perpendicular to an alignment direction of a photosensor array, so that each filter element is selectively inserted in the optical passage. The color separation filter must be stably stopped at three points in the moving direction, and must be moved by a long distance corresponding to a total of the widths of at least two filter elements. Since the moving distance of the color separation filter is large, the image reading speed cannot be improved. If the moving speed of the color separation filter is increased to improve the image reading speed, positioning precision of the color separation filter is degraded.
In the color image reading apparatus described in Japanese Patent Disclosure (Kokai) No. 61-218263, independent R, G, and B color separation filters are selectively inserted in the optical passage. However, since the installation space for the three color separation filters and three driving means for independently driving these filters must be assured, a distance from the document surface to the image sensor is increased, resulting in a bulky apparatus. The moving distance of the color separation filters is increased in accordance with a distance between the image sensor and the farthest filter from the image sensor. This is because as the filter is separated farther from the image sensor, a width of a light beam is increased, and the width of the filter element must be increased accordingly. In the apparatus described in Japanese Patent Disclosure (Kokai) No. 61-218263, since this distance is large, the width of each filter element must be increased, and hence, the moving distance of the filter is increased.